Citi Heavy Industries Co Ltd has started online subscription for its A-share initial public offering, the biggest so far this year in terms of market capitalization.

The maker of heavy machinery plans to raise around 3.2 billion yuan ($505.90 million) by issuing 685 million new shares at 4.67 yuan each.

A total of 315 million shares, or 45.98 percent, will be sold online. Online and offline issuing will end by July 2.

The IPO may put further pressure on the A-share market, where the benchmark index has recently dived below the psychologically important mark of 2,300 points.

The Shanghai Composite Index closed 0.23 percent down at 2,216.93 on June 27.